The present invention relates to a microwave heating apparatus comprising a microwave applicator, a microwave heating system including the microwave heating apparatus and a method of using the microwave heating system according to the preambles of the independent claims.
Microwave heated system for carrying out chemical reactions, and particularly organic synthesis reaction, is an important and well-known technique. Using microwave heating makes it possible to increase the reaction rate of chemical reactions with order of magnitudes. The use of microwaves also often leads to higher yield and purity of the final product.
Microwave assisted chemistry has been used for many years. However, the apparatuses and methods have to a great extent been based upon conventional domestic microwave ovens. Domestic microwave ovens have a multimode cavity and the energy is applied at a fixed frequency at 2450 MHz. The use of single mode cavities have also been reported, see e.g. U.S. Pat No. 5,393,492 and U.S. Pat No. 4,681,740.
Recent developments have led towards apparatuses comprising a microwave generator, a separate applicator for holding the load (or sample) to be treated, and a waveguide leading the generated microwave radiation from the generator and coupling it into the applicator. Even if the system consists of a TE10 waveguide using a 2450 MHz to which a magnetron generator is connected in one end and the sample container is in the other end, there is a need for a matching device in the form of at least a metal post or iris between the generator and load, in order to achieve a reasonable efficiency.
When coupling electromagnetic radiation such as microwaves from a source to an applicator, it is important to match the transmission line impedance and the applicator impedance (with load) in order to achieve a good transfer of power. However, the dielectric properties of the load may influence drastically upon the impedance of the applicator, as well as its electrical size, and the dielectric properties of the sample often change considerably with both temperature and applied frequency. Thus, an impedance mismatch between the source and the applicator will often occur and the coupling and thereby the heating process becomes less efficient and difficult to predict.
Below follows a short background description of different transmission modes used in a microwave applicator.
Consider a hollow waveguide with a given cross section that is uniform throughout its entire length. As a result of the application of these boundary conditions to the wave equation, it can be shown that only certain unique patterns for the distribution of the electric intensity E and the magnetic intensity H (taken together) can exist in the waveguide. Each unique pattern of the field distribution is called a mode. There are two types of modes possible in a hollow waveguidexe2x80x94one of them being the transverse electric (TE) mode and the other the transverse magnetic (TM) mode. In the TE mode the E has only a component transverse (that is perpendicular) to the direction of propagation, whereas the H has both transversal and longitudinal components.
In the TM mode the magnetic intensity has only a transverse component and the electric intensity has both components. Each type (TE or TM) of mode has an infinite number of sub-modes which have the common characteristics of the type to which they belong, but differs among themselves in the details of field distribution.
One of the most important characteristics of a TE or TM wave is that it has a cutoff wavelength for each mode of transmission. If the free-space wavelength is longer than the cutoff value, that particular mode cannot exist in the waveguide. For any given waveguide, the mode that has the longest cutoff wavelength is known as the dominant mode. By indexing the mode, e.g. TE01, this is indicated.
U.S. Pat. No. 4,392,039 relates to a dielectric heating applicator provided with a low-loss dielectric with a dielectric constant exceeding that of the object to be heated by microwaves. An internal resonance is excited in the applicator so that specific field pattern exists at and in the object.
According specific embodiments of the heating applicator of U.S. Pat No. 4,392,039 the dielectric is provided with an axial hole where the load can be heated (FIGS. 10-12 of U.S. Pat No. 4,392,039).
U.S. Pat No. 3,848,106 discloses an apparatus for heating by microwave energy that includes a dielectric material having low losses and a dielectric constant exceeding the dielectric constant of air. In one embodiment of this known apparatus the dielectric body is shaped as a cylindrical lining in a metal tube (illustrated in FIGS. 7 and 8 of U.S. Pat No. 3,848,106) intended for heating material with cylindrical cross-section being positioned in the tube in coaxial relationship to said tube.
The embodiment of the cylindrical dielectric material does not take into account any backwards interaction, i.e. load influence on the impedance matching conditions, or system matching.
The overall object of the microwave heating apparatus according to the present invention is to achieve a heating apparatus where the heating process is more efficient and easier to predict than in the prior art heating systems.
A further object of the present invention is to enable parallel processing of several microwave heating apparatuses arranged in a microwave heating system where each apparatus may be individually controlled with regard to temperature, time and frequency of the microwaves.
The above-mentioned objects are achieved by the present invention according to the independent claims.
Preferred embodiments are set forth in the dependent claims.
Thus, by providing a microwave applicator with the geometrical characteristics as stated in the independent claims the performance of the heating apparatus is increased.
The present invention is particularly advantageous for small load volumes. In this context a volume is considered small if it is less than 5 ml and in particular if it is less than 2 ml.
If the volume is more than about 2 ml, some technical advantages with ceramic applicator systems according to the present invention may no longer fully apply; a 2 ml load of a diameter of 10 mm becomes 25 mm high and this is so large that applicators without using a ceramic material may be used with acceptable results.
If the load volume is less than 0.4 ml it may be too difficult to control ambient influences such as cooling by a vial itself, and to measure the load temperature and pressure without disturbing it too much. A 0.5 ml load with Ø6 mm becomes 18 mm high, and another reasonably optimised load with regard to applicator design and associated sensors may have Ø9 mm, be 12 mm high and thus have a volume of about 0.75 mL.
The applicator is to be xe2x80x9csmallxe2x80x9d, to facilitate the design of compact multi-applicator systems for parallel operation. Since the applicator with load must be resonant in order for the microwave heating efficiency (the percentage of input microwave power to the applicator which is actually absorbed by the load) to become high, the characteristic size (generally: the diameter) of the applicator must be of the order of between a half and a full wavelength. At 2450 MHz this wavelength is about 120 mm in free space, so an air-filled applicator will be at least about 60 mm in diameter. By using a ceramic applicator with high permittivity instead of air, the size can be reduced by a factor of approximately the square root of the ceramic permittivity.
As an example, using a microwave ceramic with permittivity 100, the applicator diameter becomes about 17 mm (as in the preferred embodiment described below).
Below follows an overview of some considerations taken into account when developing the present invention:
There are four degrees of freedom in cases where the microwave heating apparatus is intended for heating of liquids with 0.5-1.0 ml volume using 2450 MHz microwaves. The liquids have permittivities ranging from about 10 to 70, as is the case for a vast majority of dipolar liquids over a large temperature range at this frequency.
These four degrees of freedom are: the diameter of the dielectric applicator; the permittivity of the dielectric material used in the applicator; the diameter of the load chamber and the diameter of the liquid column of the load.
The given volume then determines the height of the liquid column, and the height of the applicator is to be determined for obtaining resonance for a suitable mode.
The mode choice should be so that there can be a minimum deviation in the angular and radial variations of the heating pattern. Hence, only the lowest-order TM01, type mode is possible in that it has no angular variation of the fields. The vertical (axial) mode index cannot be zero due to the magnetic wall effect at the top. Therefore the next lowest type is to be considered. It has two quarter-wave periods and is thus the TM011 mode. The pattern is of course distorted in comparison with the pattern in a homogenous dielectric body. If the permittivity of the dielectric material is very high, a large part of the energy will oscillate in it and the field distribution in the hole will be close to that of the normal open-ended TM011-mode.
According to the present invention the load diffraction is actively used to improve the overall impedance matching of the system when the permittivity and the loss factor of the load substance are varied. By using the present invention it is not required to perform any physical changes to the system for different loads.
The present invention provides a microwave feed sub-system, being the lower section of the dielectric applicator, which produces a stronger coupling to the upper section (provided with the load chamber) when the load has a low permittivity and provides a negative feedback to the upper section so that the coupling varies little when the load permittivity varies. This is achieved, according to a preferred embodiment of the present invention, by both the high permittivity ceramic dielectric material surrounding the load at close distance, and by arranging the lower section below the load, wherein the lower section is a solid cross-section, with an axial rather than a radial microwave feeding position.
There is a normal propagating mode in the cylindrical part of the applicator with load. The basic field pattern of the coaxial TE mode in the feeding section at the bottom is the same as for the TM01 mode. In the embodiments of the present invention provided with a conical section between the microwave coupling means and the upper section, there will be an evanescent propagation of the mode, but since the axial length is short the coupling can be optimised. This embodiment of the invention results in the coupling factor of the applicator for varying load permittivity to become significantly less than otherwisexe2x80x94a favourable negative feedback is accomplished and results in maintaining a high efficiency of the system for variable loads.
The high dielectric properties of the ceramic applicator make it possible to construct a small applicator, that gives no radiation of microwaves and it also makes it semi-resonant since it will xe2x80x9cbufferxe2x80x9d the changes in the sample""s dielectric properties and hence make the necessary bandwidth of the microwave generator smaller.
By arranging a predetermined number of microwave heating apparatus according to the present invention parallel processing of chemical reactions is achieved.
This is one major advantage with the dielectric applicator according to the present invention because it makes it possible to achieve the compact support structure that is a necessary condition for parallel processing.
Another major advantage is that by mounting the dielectric ceramic applicator in a thick-walled metal tube in combination with an effective pressure sealing in the upper end and the solid ceramic section at the bottom the applicator is effectively pressure sealed.
According to a further preferred embodiment of the present invention one or many small holes is/are arranged in the applicator into the load chamber, essentially in a radial direction, for pressurizing purposes, for temperature monitoring and also in order to be able to perform quick cooling by forced gas or air venting.
According to still another preferred embodiment of the invention a semiconductor based microwave generator is used. This makes it possible to vary the frequency and hence get a higher efficiency of the microwave heating system. The variation of the dielectric properties of the sample gives different resonance frequencies for the different samples and this has to be compensated for. The variation of the frequency can compensate for this and by using a semiconductor based microwave generator no mechanical tuning devices are needed. The semiconductor based microwave generator can also become very small, which makes it possible to have a number of microwave generators in the same system/instrument. No high voltage system is needed as is the case for a magnetron based system. dr
FIGS. 1a and 1b show longitudinal sectional schematic illustrations of the microwave heating apparatus according to the present invention.
FIGS. 1c and 1d show schematic illustrations of the microwave heating apparatus according to the present invention in a cross-sectional view of the upper and lower sections, respectively.
FIGS. 2a and 2b show longitudinal sectional schematic illustrations of a preferred embodiment of the dielectric applicator according to the present invention.
FIG. 2c shows a schematic illustration of a preferred embodiment of the dielectric applicator according to the present invention in a cross-sectional view.
FIGS. 3a-3e show longitudinal sectional schematic illustrations of variations of the preferred embodiment of the dielectric applicator according to the present invention along line Axe2x80x94A in FIG. 2c. 
FIGS. 4a-4e show longitudinal sectional schematic illustrations of various embodiments of the microwave applicator according to the present invention.
FIG. 5 schematically illustrates a microwave heating system where the microwave heating apparatus may be used.